1851.] OF THE ROYAL INSTITUTION. 11 
where one begins life as a burrower or a crawler, and is converted into 
an animal of rapid and powerful flight. 
Most insects quit the egg in the form of a worm, which masking, 
as it were, a different and higher form, is called the ‘larva; ’ it is active 
and voracious — but usually falls into a kind of torpor, during which 
the changes take place which issue in the flying insect; during the 
passive stage of metamorphosis it is called a ‘ pupa;’ the last volant 
stage is the ‘ imago.’ 
The chief steps in the metamorphosis were traced as they affect the 
outward form, the digestive organs, the circulatory, and respiratory, 
and nervous systems. 
The main differences in the metamorphoses of insects relate to the 
place where, and the time during which they are undergone. The 
young cockroach and the little aphis, which were first acephalous 
and apodal, and then had thirteen equal segments, with soft un- 
jointed legs, proceed to acquire a distinct head with antenne, a 
thorax with three pairs of long jointed legs, and an abdomen, before 
they quit the egg; they thus enter upon active life under the guise 
of a crab, instead ofa worm. With regard to the Aphis, that insect, 
instead of proceeding to perfect its individual development, may at 
once begin the great business of its existence by parthenogenetic 
procreation. Bonnet’s experiments, which first brought to light this 
marvellous fact, have received uniform confirmation from all subse- 
quent enquirers, and no natural phenomenon is now better deter- 
mined. 
From seven to eleven successive generations have been traced 
before the individual has finally metamorphosed itself into the 
winged male or winged oviparous female. 
In Autumn, when the nights grow chilly and long, the oviparous 
imago completes her duty by depositing the eggs in the axils of the 
leaves of the plant, where they are protected from the winter frost, 
and ready to be hatched at the return of Spring. Then recom- 
mences the cycle of change, which being carried through a succes- 
sion of individuals and not completed in a single life-time, is a 
‘ metagenesis ’ rather than a ‘ metamorphosis.’ 
This phenomenon which, until very recently was deemed an 
exception, and a most marvellous one, in Nature, now proves to be 
an example of a condition of procreation to which the greater part 
of organised Nature is subject. 
The Lecturer was inevitably limited in his choice of illustrations : 
and proceeded to an instance of metagenesis from the radiated sub- 
kingdom of animals. 
The stages of this metagenesis have been best and most completely 
traced in the Medusa aurita, by Siebold, Dalyell, Sars, and others. 
The first step was made by Siebold who,*in 1839, traced the 
development of the Medusa aurita from the egg to a stage resembling 
